Resistance to viral infection involves activation of the innate immune system. An essential component of this response is activation of an alarm system that signals the presence of an invading pathogen. This system includes induction of inflammatory mediators, such as interleukin 1 and tumor necrosis factor, and antiviral products such as type I interferon (IFN) which in turn activates a cascade of cellular gene products involved in antiviral defense, some of which are also directly induced in response to virus infection. These induced proteins activate a cellular antiviral state capable of inhibiting diverse viral infections through a wide variety of mechanisms. While the signaling cascade and transcriptional mechanisms involved in activation of cellular genes in response to IFN treatment have been recently characterized, the mechanisms responsible for the initial induction of the IFN genes themselves and the signals that set this process in motion following detection of an active viral infection remain unclear. The type I IFN gene family is composed of more than a dozen genes that are divided into two subfamilies, alpha and beta, with the alpha subfamily consisting of at least two groups displaying distinct expression patterns (early and delayed) represented in the mouse by IFNalpha4 (early) and IFNalpha6 (late). While all IFN genes are induced in response to virus, the mechanisms, kinetics, and cell-type specificity of induction are distinct. A shared component that nonetheless is responsible for some aspects of these distinct expression patterns is the transcription factor IFN regulatory factor 7 (IRF7). IRF7 is essential for the expression of IFNalpha6 and modulates the expression of IFNalpha4 while playing only a minor role in induction of IFNbeta. The activity of IRF7 is controlled at the level of protein abundance, subcellular compartmentalization, DNA binding, and transactivation, all of which are altered by viral infection. Its central position in the activation of IFN genes and its direct response to viral infection make it an ideal target to elucidate the signaling pathways initiated by virus and the cellular and transcriptional control mechanisms involved in innate immune gene induction. This proposal will characterize the structural and functional attributes of IRF7 and its cousin IRF3 that contribute to the specificity of IFN gene induction and will delineate mechanisms of virus-induced regulation of IRF7 and the cellular signaling pathway that detects and responds to viral infection. These studies should reveal important features of transcriptional regulatory mechanisms and uncover the nature of the antiviral alarm system.